Optical disks have become very popular as optical information storage media to read data from, and write data to, by optical techniques. In optical disk technologies, data can be read out from a rotating optical disk by irradiating the disk with a relatively weak light beam with a constant intensity, and detecting the light that has been modulated by, and reflected from, the optical disk.
On a read-only optical disk, information is already stored as pits that are arranged spirally during the manufacturing process of the optical disk. On the other hand, on a rewritable optical disk, a recording material film, from/on which data can be read and written optically, is deposited by an evaporation process, for example, on the surface of a base material on which tracks with spiral lands or grooves are arranged. In writing data on such a rewritable optical disk, data is written there by irradiating the optical disk with a light beam, of which the optical power has been changed according to the data to be written, and locally changing the property of the recording material film.
It should be noted that the depth of the pits, the depth of the tracks, and the thickness of the recording material film are all smaller than the thickness of the optical disk base material. For that reason, those portions of the optical disk, where data is stored, define a two-dimensional plane, which is sometimes called an “information storage plane”. However, considering that such an “information storage plane” actually has a physical dimension in the depth direction, too, the term “information storage plane” will be replaced herein by another term “information storage layer”. Every optical disk has at least one such information storage layer. Optionally, a single information storage layer may actually include a plurality of layers such as a phase-change material layer and a reflective layer.
To read data that is stored on a recordable optical disk or to write data on such an optical disk, the light beam always needs to maintain a predetermined converging state on a target track on an information storage layer. For that purpose, a “focus control” and a “tracking control” are required. The “focus control” means controlling the position of an objective lens perpendicularly to the information storage layer such that the focus position of the light beam is always located on the information storage layer. On the other hand, the “tracking control” means controlling the position of the objective lens along the radius of a given optical disk such that the light beam spot is always located right on a target track.
Various types of optical disks such as DVD (digital versatile disc)-ROM, DVD-RAM, DVD-RW, DVD-R, DVD+RW and DVD+R have become more and more popular these days as storage media on which a huge amount of information can be stored at a high density. Meanwhile, CDs (compact discs) are still popular now. Currently, next-generation optical disks, including Blu-ray disc (BD), which can store an even greater amount of information at a much higher density, are under development, and some of them have already been put on the market. Furthermore, optical disks with a plurality of information storage layers, which are stacked one upon the other to increase the amount of data that can be stored in a single optical disk, have also been developed.
The light beam to irradiate the information storage layer of an optical disk is emitted from a semiconductor laser diode as a light source. If an optical disk is irradiated with a laser beam emitted from a semiconductor laser diode, then part of the laser beam that has been reflected from the optical disk will enter the semiconductor laser diode. Such a returning beam will decrease the stability of laser beam emission from the semiconductor laser diode, thus increasing noise components in the laser beam intensity and preventing the information stored on the optical disk from being read accurately. This is a problem.
As a means for solving such a returning beam problem, a drive method in which RF current with a frequency of 250 MHz to 350 MHz is superimposed on the drive current for the semiconductor laser diode (which is called an “RF current superimposition method”) has been adopted. An optical information read/write apparatus that adopts the RF current superimposition method is disclosed in Patent Documents Nos. 1 and 2, for example.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 07-93794 (see Paragraphs #0016 through #0048 and #0063 through #0065 and FIGS. 1 and 5)        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 07-320292 (see Paragraphs #0035 through #0064 and #0075 through #0094 and FIG. 1)        